Animal models created by transgenic and gene knockout technology are invaluable in evaluating gene function in vivo and in determining how certain genes regulate cell growth and differentiation and how alterations in these genes contribute to cancer. Mice are genetically modified in this facility either by pronucleus injection of DNA into fertilized eggs, by injection of retroviruses into the perivitteline space of fertilized eggs, or by injection of mutated embryonic stem cells (ES-cells) into blastocysts. The objective of this shared resource is to provide genetically modified mice to principal Investigators within the Cancer Center in the most time- and costeffective way. In addition, the availability of these mouse strains greatly stimulates interactions among investigators at St Jude.